chore: upgrade @primer/react to v38

[Miablo]

Pull Request

• Upgrades @primer/react 36.27.0 → 38.39.0 and @primer/octicons-react to 19.35.0
• Primer v38 removes the library's dependency on styled-components, which drops support for the sx prop and styled-system props — migrates every affected component off sx to CSS Modules
• New .module.css files added alongside each migrated component, plus a shared src/app/styles/shared.module.css for common auth-card styling
• No component behavior changes — this is a styling-mechanism migration only

Addresses the @primer/react v38 step of the dependency upgrade plan in #412.

Working UI post-changes

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request
• run npm run format and fix any formatting issues that have been introduced
• run npm run lint and fix any linting issues that have been introduced
• run npm run test and run tests